1. Field of the Invention
The invention is in the field of electronics, and more specifically in the field of differential mode electronics.
2. Related Art
FIG. 1 is an illustration of a low input impedance Current Mirror 100 of the prior art. Current Mirror 100 operates by using a feedback loop to assure that an output current is proportional to an input current. A constant bias voltage is applied to the gate of Transistor 115. The current through Transistor 115 depends on the voltage difference between the gate and the source. A current through a Transistor 120 is dependent on a voltage on the gate of Transistor 120. The voltage at a Node 150 is held constant by a negative feedback loop between Transistors 115 and 120. If the voltage at Node 150 rose then the current through Transistor 115 would decrease. However this current fall would increase the voltage at Node 125. The voltage at Node 125 is applied to the gate of Transistor 120, and thus the increase of voltage at Node 125 would cause an increase on current flow through Transistor 120. The increase in current flow through Transistor 120 increases the gate-to-source voltage of Transistor 115. Since the gate voltage Vbias of Transistor 115 is held constant the voltage at Note 150 falls and completes the negative feedback.
Transistor 120 and a Transistor 130 share a common gate voltage. The current through each is, therefore, dependent on the voltage at Node 125. If Current Source 145 is matched to Current Source 110, and Transistors 120 and 130 have the same quiescent current and are built from matched unit devices, a current at an Output 135 must be essentially proportional to a current at an Input 140.
Current (I120) through Transistor 120 will be the sum of the input current Iin and the current I110 provided by Current Source 110, (I120=Iin+I110). Likewise the current (I130) through Transistor 130 will be the current (I145) provided by Current Source 145 minus the output current (Iout), (I130=−Iout+I145). Because of these relationships, when I120=I130, Iout=−Iin. Current amplification can be achieved by selecting the various ratios of the transconductances of Transistors 120 and 130. The Current Mirror 100 is referred to as a gm-boosted current mirror because the transconductance (gm) of Transistor 115 is boosted by the gain at Node 125, which stabilizes the voltage at Node 150 and thus creates a very low impedance input.
FIG. 2 is an illustration of a Pseudo-Differential Transconductor 200 of the prior art. This circuit operates on the same general principals as Current Mirror 100 discussed in reference to FIG. 1. However, to achieve a differential output a mirrored pair of circuits is used. Mirrored elements of these circuits (and other mirrored circuits discussed herein) are referred to herein by a numeric value and the numeric value prime (′). As used herein the term “mirrored” is used to refer to components, typically having essentially the same characteristics, disposed on opposing sides of a differential circuit, each of the opposing sides being configured to process one side of a differential input signal. Specifically, a voltage difference between an Input 205 and an Input 205′ is reproduced between Nodes 210 and 210′ using a negative feedback loop including an Input Transistor 215 (215′) and a differential Amplifier 220 (220′). Any difference between the voltages at Input 205 and 205′ and, thus, Node 210 and Node 210′, results in a current (IR) through a Resistor 225. Output currents Ioutp and Ioutn at Nodes 240 and 240′ are equal to a current (I230) from a Current Source 230 minus a current I235 into a Current Sink 235, and minus IR, (I230−I235−IR=Iout). The sign of IR is dependent on which way current flows though Resistor 225, thus, if I230 equals I235, Iout and Iout′ will be equal in magnitude but opposite in polarity.
To build a linear transconductor it is common to boost the gm of a transistor by means of feedback loops. The boosted gm is used to generate a smaller (but better controlled) actual gm of the circuit. In FIG. 2, the controlled actual gm of the circuit is provided by the Resistor 225 (gm=1/R225, where R225 is the resistance of Resistor 225). A wide variety of methods of manipulating the gm of a transistor or a transconductor are known in the art and that of circuit of FIG. 2 is but one example of these methods. A transistor or other component is considered gm-boosted when a circuit is used to manipulate (raise or lower) its effective gm. The circuit used to manipulate the effective gm is referred to as a gm-boosting circuit.
Pseudo-Differential Transconductor 200 is not fully differential because some components are not differential with respect to the two sides of the mirrored circuit. For example, Amplifier 220 is not differential with respect to the input voltages Vinn and Vinp, although Amplifiers 220 and 220′ are differential with respect to their own inputs. It is common that components of a differential circuit configured to manipulate the gm of transistors cause the circuit to be only pseudo-differential rather than fully differential because each input or input node is independently gm-boosted and/or the current to do so has not come from a common source. As used herein the term “fully-differential” is meant to indicate a circuit in which those components used to boost or otherwise manipulate the effective gm of the circuit are themselves differential with respect to the differential inputs of the circuit and optionally also differential with respect to signals internal to the circuit, e.g., differential with respect to each side of the mirrored circuit.